Skeletons protect and support animals' bodies and serve as anchoring points for their muscles. They can be external (exoskeleton) and made of cuticle, as in arthropods such as insects and crustaceans, or internal (endoskeleton) and based on bones or cartilage, as in the phylum Chordata (eg the vertebrates). In spite of their many advantages, skeletons have drawbacks. They are rigid and thus limit an animal's movements. Indeed, skeletons allow movement only at specific points - the joints.
Some invertebrates, notably the soft-bodied molluscs, have evolved without a rigid skeleton. Octopi (see inset picture), probably the most developed and intelligent molluscs, have had to find different solutions for the problems of protection and posture. Their soft bodies allow them to pass through small holes and crevices in their rocky habitats to find shelter and prey. In addition, they can rapidly change skin colour, pattern and texture, creating an exceptional camouflage, and they can even release a cloud of ink to confuse a potential predator.
Octopi can maintain posture partly because the buoyancy of sea-water supports most of their body weight. Principally, however, they do it with a unique muscular arrangement, which is especially noticeable in the arms.
Three muscle groups (longitudinal, transverse and helical) all work against each other to create a dynamic framework, which maintains shape and posture and gives the arm the skeletal stiffness required for generating movement.
This arrangement allows longitudinal muscles, which run longitudinally along the arm, to shorten and thicken the arm. In contrast, the transverse muscles, lying perpendicular to the longitudinal ones, make the arm thinner and thus elongate it. The arm can be twisted by the helical muscles arranged at the outer part of the arm.
By combining these basic movements, an octopus can perform a vast variety of complex movements with virtually an infinite number of degrees of freedom (ie it has many alternative ways to perform the same task). This is much more than the minimum of six degrees of freedom required to manipulate an object in three-dimensional space (our hand has seven degrees of freedom). This "hyper-redundancy" allows the octopus to move, forage and escape from predators in very complex habitats that are full of obstacles, such as coral reefs. This complex movement system is enhanced by suckers, distributed along each of the eight arms (see main picture), endowing a remarkable grip. Together, these features allow the octopus to perform sophisticated and fine manipulative tasks, such as untying surgical threads and unscrewing jars etc.
At the Hebrew University of Jerusalem, we study the octopus because it is an excellent example of a successful evolutionary solution to the complex engineering problem of creating and controlling motion in flexible structures. Studying the arm's bio-mechanics and the strategies used by the octopus nervous system to control it will help us build and control flexible robots to serve in rescue missions in constrained environments, such as ruins, atomic reactors and the seabed, which are inaccessible to humans or to the currently available jointed robots. Small-scale versions of such robots may also be able to penetrate complex structures in our bodies and perform surgical operations.
Additional reporting by Yoram Yekutieli and Binyamin Hochner, from the Hebrew University of Jerusalem